The invention relates to packet-switched data transmission and particularly to packet-switched data transmission in a telecommunication network of a digital mobile telephone system when data is transmitted at various rates over the air interface of the mobile telephone system.
In most digital mobile telephone systems the carrier wave of the radio path is divided between several users by using, for example, a TDMA (Time Division Multiple Access) or CDMA (Code Division Multiple Access) method. The common feature of the methods is that a call is encoded as sequences of a certain length into blocks to be transferred in frames on the radio path. The blocks comprise channel coding information of a standard length, a payload, and error correction information, whose length is determined by the error correction level used. Particularly for data calls, various error correction levels are defined. The length of the payload to be transferred in a block on the ratio path is thus dependent on the error correction level used in the call and varies from call to call. Usually only the payload of a block is transferred in the transmission network, the payload being characterized in that the payloads of one and the same call are equal in length, but the payloads of two different calls can be different in length. A similar situation can also be encountered in other telecommunication systems in which a protocol transferring information over an interface uses payloads of different lengths. The following is a description of the problems involved in the prior art. Particular attention is paid to mobile systems, but the invention is not to be restricted to them.
Packet-switched transmission networks are often based either on ATM technology (Asynchronous Transfer Mode) or HDLC protocol (High-level Data Link Control).
In the asynchronous mode ATM information is transferred in standard-length 53-byte ATM cells according to FIG. 1A: in each cell 5 bytes form the Header HE of the cell and the remaining 48 bytes contain the actual Information Field IF. The fields of the cell are transmitted from left to right. To simplify, the user information to be transferred is cut in the ATM technology to sequences of a standard length, and each sequence is positioned in the information field IF of the ATM cell. If the amount of user information is less than 48 bytes, padding bits are transmitted in the ATM cell. The problem in the ATM technology is that the payload transferred on the radio path does usually not measure 48 bytes, i.e. 384 bits, wherefore padding bits are needed and the transmission capacity is underused. In addition, if the payload in a block on the radio path is longer than 48 bytes, the content of one block must be transmitted in several ATM cells. This may impair the quality of a connection in a loaded network, and the transmission capacity may be underused when a radio packet is late for its transmission slot because, for example, due to the load the last ATM cell belonging to the block arrives too late at the network node to be transmitted onto the radio path. All the bits belonging to the block must have arrived before the block to be transmitted onto the radio path can be encoded.
Another problem in the ATM technology is that the addition of padding bits and/or division of the payload between a plural number of cells adds to the processing of the information to be transferred when the transmission network is entered or exited.
The use of one standard-length cell or packet which is based on the ATM technology and the length of whose information field has been defined to equal the length of the payload transmitted on the radio path makes it possible to reduce the use of padding bits. It remains to be a problem, however, that when the length of the payload to be transmitted on the radio path is different, padding bits must still be used and/or the payload must be divided between a plural number of packets.
A packet transmission mechanism based on the High Level Data Link Control protocol HDLC applicable to synchronized transmission is Frame Relay FR, which is used, for example, in the General Packet Radio Service GPRS. The GPRS is one of the items being standardized in phase 2+ of the Pan-European GSM system (Global System for Mobile Communications) in the ETSI (European Telecommunications Standards Institute). FIG. 1B shows the basic structure of an FR frame according to the HDLC protocol without any start/end bits (for asynchronous links) or bits or octets positioned in the frame on account of transparency. The frame comprises five fields. The fields of the frame are transmitted in order from left to right. The frame begins and ends with a 1-octet-long Flag F, which is a predetermined bit sequence that starts and ends with a xe2x80x980xe2x80x99 bit and contains six xe2x80x981xe2x80x99 bits in between. The flag starting the frame is followed by an Address field AD and then by an Information field I. The information field I is not of a fixed length, but it contains an integer number of octets of transferable information and padding bits. The information field is followed by a 2-octet Frame Check Sequence FCS.
The HDLC, and thereby the FR, contains a so-called bit stuffing protocol. The occurrence of certain bit patterns, such as the content reserved for the flag, in the information field is prevented by the addition of padding bits. The padding bits, however, pose problems in the transmission network. In some mechanisms based on the HDLC, the payload transferred is always of a certain length, whereby the length of the frame is bound to the information content to be transferred. In a network employing the FR technology the parties participating in the transfer can negotiate the length of the frame to be used before the transfer. If the length of the frame and thereby the length of the information field are constant, the length of the payload transferred in the frame varies with the information content to be transferred. For example, the payload of the first of two successive blocks of one and the same call transmitted on the radio path can fit into one frame, whereas the payload of the second block may have to be divided between a plural number of frames, although the payloads on the radio path are equal in length. If the length of the frame has been determined to be such that no division is necessary, padding bits must be used, the number of padding bits varying from one frame to another, completely depending on the information content to be transferred.
The problem in the HDLC and thereby in the FR technology is the processing need caused by the padding bits when the network is entered or exited, and the wasted transmission capacity. In addition, the variation in the frame length causes variation in the transfer times. Likewise, the variation in the length of the payload transferred in the frame causes variation in the transfer times of the payload. If the transfer times vary too much, the quality of the call will be impaired. In real-time speech transmission in particular, it is important that the transfer times are as constant as possible for the duration of the entire speech item. Further, variation in the transfer times of data call packets is also disadvantageous in mobile telephone systems, since the blocks must be transmitted onto the radio path at a constant rate in accordance with the radio path protocol, irrespective of their content.
The object of the present invention is to provide a method and a telecommunication system by which the above problems can be overcome and the efficiency of the transmission capacity of a packet-switched transmission network can be maximized and the payload processing required by the transmission network can be minimized. The object is achieved with a method of the invention used for data transmission in a telecommunication system that comprises at least one packet-switched transmission network in which information is transferred in information packets of a standard length, the number of said standard lengths having been determined in advance. The method of the invention is characterized by maintaining a unique length code for each standard length, and adding the length code indicating the length of the packet to the header of the packet to be transferred.
The invention also relates to a system in which the method of the invention can be used. The system is a telecommunication system utilizing wireless data transmission, the system comprising a packet-switched transmission network for transmitting information in standard-length data packets, the number of the standard lengths having been determined in advance, and the transmission network comprising at least a first node for transmitting data packets to the transmission network and a second node for receiving data packets from the transmission network, said first node comprising reception means for receiving information arriving at the transmission network, modifying means for assembling data packets, and transmission means for transmitting the data packets to the transmission network, and said second node comprising at least reception means for receiving data packets. The system is characterized in that the modification means are arranged to provide the data packet with a header, the header comprising at least a length code consisting of a bit sequence to indicate the standard length used in the packet, and the reception means of the second node being arranged to detect the length of the data packet received from the length code contained in the header of the packet.
The invention is based on the idea that the length of the packets can be indicated by a short bit sequence when packets of a standard-length are used, the lengths of the packets being predetermined.
The advantage of the method and arrangement according to the invention is that packets of different lengths can be transmitted in the transmission network without any padding bits, using a short header. There is thus little redundance in the information to be transferred. Two different packet lengths can be indicated by a 1-bit length code, four by a 2-bit code, eight by a 3-bit code, etc., irrespective of the actual length of the packet. When the packet lengths to be used are selected suitably, no padding bits need to be used.
In a preferred embodiment of the invention, as many packet lengths are defined as there are net data transmission rates over the air interface. The advantage is that a unique packet length can be defined for each payload transferred over the air interface, and with the help of these packet lengths the use of the transmission network can be optimized.
In another preferred embodiment of the invention, the packet lengths are defined such that the payload transferred in them is equal to the payload transferred over the air interface. The advantage is that the payload transferred over the air interface can be positioned in a packet without adding any padding bits and without dividing the payload, for example, into two smaller packets. This shortens the transfer times and enhances the utilization of the transfer capacity.
In a preferred embodiment of the invention, error correction information is added to the packet in accordance with the error correction level to be used over the air interface. The error correction here means that an error is detected and optionally corrected in accordance with the error correction protocol used in the embodiment. The advantage is that the checking and correction of the transmission errors of the transmission network correspond to the error correction of the air interface, which is notably more susceptible to errors. If information is transferred over the air interface without error correction, it can also be transferred in the transmission network without error correction.
In another preferred embodiment of the invention, header check data is added to the header of the information packet, and the forwarding of a packet received is started in a transmitting network node once the header has been checked. The advantage, is that the forwarding of the packet can be started before it has been received in its entirety, and so the transfer time of the packet can be shortened. The checking of the correctness of the header ensures that the packet is transmitted to the correct address. This enhances the degree of use of transmission network capacity, since packets with an erroneous address will not be forwarded.
In a preferred embodiment of the invention the data packets containing user information are separated from the control packets on the basis of a bit in the header, and a piece of information indicating the length of the packet is added to the control packet. The advantage is that data and control packets can be transmitted on the same transmission connection, and that the lengths of the control packets may vary and that the signalling information need not be adjusted in accordance with the lengths of the data packets. Usually, control packets are transmitted seldom, and they are short. In the present embodiment, no padding bits need to be used. In addition, the embodiment allows the transmission of even very long control packets in the transmission network, for example, when the network is being updated.
The preferred embodiments of the method and arrangement according to the invention appear from the attached dependent claims 2 to 6 and 8 to 12.